1. Field of the Invention
This invention relates to packaging techniques and especially to techniques for making vacuum or gas-filled packages such as those used to contain food products. In one important aspect, this invention relates to improved packaging apparatus adapted to form top closures which conform substantially to the shape of the product surface. The disclosed apparatus moreover advantageously can make a variety of different kinds of packages with only minimal alteration of the equipment to change over from one type of package to another.
2. Description of the Prior Art
Various kinds of packaging machines have been proposed and used over the years. Commonly the automatic packaging machines in commercial use make vacuum packages from two continuous webs of plastic film supplied as rollstock, one web being formed into cup-like containers with flanges around the mouth, the other serving to provide top closures which are sealed to the cup flanges. Typically, the lower web is first thermoformed into successive container cups, and thereafter the cups are advanced together with the upper web through one or more packaging stations where (1) the top is sealed to the cup part-way around its periphery while leaving an evacuation opening, (2) the package is evacuated through the opening, and (3) the evacuation opening is closed off to completely seal the package from atmosphere.
For certain types of packages, especially those made with receptacle cups of semi-rigid material where the upper product surface is below the plane of the cup flanges, it is desirable to stretch-form the top material so that it fits into the cup, somewhat in telescoping fashion, to engage the product surface and thereby permit the product to carry the stress load of atmospheric pressure. There are various ways in which such formed tops have been made, as described hereinbelow.
In one packaging machine, shown in U.S. Pat. No. 3,545,163 issued to R. A. Mahaffy, et al, semi-rigid cups formed in the lower web of plastic film move through three successive packaging stations together with the upper web. In the first station, an "initial" or partial seal is made between the upper web and the peripheral flange around the mouth of each corresponding cup; at the same time, heat is transferred to a part of the upper web inboard of the seal line, to soften the plastic for subsequent stretch-forming. In the second station, the heat-softened portions of the upper web are stretched upwardly, away from the cup, to make a top having the appearance somewhat of an upside-down cup, and having a depth approximately equal to the distance between the flange plane of the product-containing cup and the upper surfaces of the product in the cup; the second station also included means to chill the stretch-formed upper web while held in its stretched condition, thereby to prevent shrink-back of the plastic. The formed webs then are shifted to the third station where the formed top is inverted and forced down into the product cup to be pressed against the top surface of the product; also at this station, the package is evacuated, and a final seal made at the evacuation opening.
It will be clear that the shape of the formed top in the apparatus described above is effectively controlled by the top-forming recess in the second station. When used with products having a relatively flat surface, such as sliced luncheon meats, a top formed in such apparatus can provide reasonably close conformity to the product shape, especially when the top material is thin and flexible. However, where the top web is relatively thick and/or stiff, e.g. semi-rigid plastic, or where the top contour of the product is substantially irregular, it is not readily possible to obtain the desired excellent conformity between the top and the product surface. The machine described above also is not well adapted for ready interchangeability between different product types, since it uses a series of trays to carry the semi-rigid cups through the packaging sequences, and such trays cannot easily be altered to suit various product types.
U.S. Pat. No. 3,805,486 shows a later machine which also uses the technique of forming the top in a station preceding the evacuation-and-final-seal station. In this machine, the top is formed from the lower web of (flexible) plastic, and the product is placed on the formed top prior to its assembly to the cup formed in this case from the upper web. The lower web is conveyed through the operating stations by edge clamps rather than by trays as in the previously-described machine.
In another machine, shown in U.S. Pat. No. 3,695,900, the top is formed in the final-seal station. The preceding station serves the usual initial-seal function, and also provides for transfer of heat to the upper web to prepare it for subsequent stretch-forming. The final-seal station includes the usual vacuum chamber which serves to evacuate the package prior to making the final seal. When the vacuum chamber is vented, atmospheric pressure will force the heated top web down against the product. In this manner, the upper web of the package can be made to conform somewhat to the product profile.
However, the arrangement described in that patent is not fully satisfactory for a variety of product types. In particular, the heated plastic of the top may shrink back to set up stresses in the package tending in certain applications to physically distort the package, e.g. especially with semi-rigid container cups of only moderate thickness or stiffness. Moreover, since the force which presses the top down against the product is proportional to the difference between atmospheric pressure and the pressure in the package, it will be evident that the top-forming technique disclosed in this patent is not well suited for use in making gas-filled packages. Still another limitation with this technique is that it sometimes has a tendency to produce thinning-out or puncturing of the plastic in the regions where stretch-forming is effected.
U.S. Pat. No. 3,972,155 shows another top-forming arrangement wherein the top formation takes place in the final-seal-and-evacuation station. FIGS. 4 and 6 of that patent show the use of a plug fixed to the roof of the vacuum chamber, to press down against the film while the chamber is being closed prior to evacuation of the package. Subsequent venting of the chamber applies pressure to the top web to force that web down towards the product.
It also is known in the art to pre-form the top closure in a separate operation, to apply such formed top to the container cup in an evacuation chamber, and then vent the chamber. Although this approach has been effective for certain applications, it requires very close product control and tooling for each individual package shape, since all of the elements (i.e. the receptacle cup, the product, and the formed top) must fit exactly, in a custom-tailored arrangement for each particular product. Thus this method is costly to carry out, because of its close tolerance requirements, and also lacks versatility since each product shape must have its own unique tooling.